1. Field of the Invention
This invention relates to a process for preparing polyarylene sulfides and in particular to a process for preparing increased molecular weight polyarylene sulfides.
Polyarylene sulfides have excellent heat and chemical resistance properties and hence, have attracted a good deal of attention as materials useful for producing parts of electrical and electronic instruments, automative parts and the like. Polyarylene sulfides may be formed or molded into various molded parts, films, sheets, fibers and the like, for example, by injection or extrusion techniques. These formed or molded products have been used in applications where high heat and chemical resistance properties are required.
2. Prior Art
A typical method for production of polyarylene sulfides is known, for example, in Japanese Patent Publication (KOKOKU) No. 45-3368, which method comprises reacting a dihaloaromatic compound with an alkali metal sulfide such as sodium sulfide, in an organic amide solvent such as N-methyl pyrrolidone. However, polyarylene sulfide polymers have such a low molecular weight that they, as such, can not be successfully or satisfactorily employed in molding processes, such as an injection molding process. Therefore, it has been proposed to crosslink low molecular weight polyarylene sulfide by thermally oxidizing the material in air, thereby producing a polymer product of an increased molecular weight that may be used in molding and shaping applications. Nevertheless, certain difficulties have been experienced with the increased molecular weight polymers, when they are used in preparation of films, fibers and the like by extrusion techniques. The poor extrudability appears to be due to the occurrence of a significant level of crosslinking and branching in the polymers.
As another approach, it has been proposed to increase the molecular weight of polyarylene sulfide polymers by means of special polymerization techniques. A typical example of the polymerization processes for that purpose is disclosed in Japanese Patent Publication (KOKOKU) No. 52-12240, which is characterized by conducting the polymerization process in the presence of a polymerization aid RCOOM, wherein R is a hydrocarbyl group and M is an alkali metal. The thus resulting polymer products shown an excellent extrudability and are adaptable for use in production of films, fibers and the like. However, the process requires that the polymerization aid should be added in a molar proportion approximately equal to that of an alkali metal sulfide which is one of the reactants used in the process. If it is desired to further increase the molecular weight of the polymer, then an expensive aid such as lithium acetate or sodium benzoate should be used in an increased amount. This adds to the production cost of polyarylene sulfide and would be commercially disadvantageous. Furthermore, the process may present an environmental pollution problem, since the polymerization stage is followed by a product recovery stage from which an aqueous effluent containing organic acids and other substances is discharged. In order to prevent environmental pollution, the effluent must be subjected to a good deal of expensive treatments. Therefore, this approach is economically disadvantageous.
It has been also proposed to conduct a process for preparation of polyarylene sulfide in the presence of a large amount of deliberately added water, for example, in Japanese Patent Public Disclosure (KOKAI) No. 61-7332. However, the process requires a very prolonged reaction time and hence has low productivity. Further, since the boiling point of water is substantially lower than the temperature at which the polymerization can proceed, an exceedingly high pressure must be maintained in the reaction system. Furthermore, the considerably large specific heat of water consumes a great deal of energy in heating the reaction system up to the reaction temperature. The above-discussed productivity, reaction pressure and energy consumption problems lead to increased production costs thus making the process commercially disadvantageous.